staging: fbtft: Do not hardcode SPI CS polarity inversion

The current use of the mode flag SPI_CS_HIGH is fragile: it
overwrites anything already assigned by the SPI core.

Assign ^= SPI_CS_HIGH since we might be active high
already, and that is usually the case with GPIOs used
for chip select, even if they are in practice active low.

Add a comment clarifying why ^= SPI_CS_HIGH is the right
choice here.

Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191204233230.22309-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/fbtft/fb_uc1611.c b/drivers/staging/fbtft/fb_uc1611.c
index e763205..f61e373 100644 (file)
--- a/drivers/staging/fbtft/fb_uc1611.c
+++ b/drivers/staging/fbtft/fb_uc1611.c
@@ -63,11 +63,17 @@ static int init_display(struct fbtft_par *par)
 {
        int ret;
 
-       /* Set CS active high */
-       par->spi->mode |= SPI_CS_HIGH;
+       /*
+        * Set CS active inverse polarity: just setting SPI_CS_HIGH does not
+        * work with GPIO based chip selects that are logically active high
+        * but inverted inside the GPIO library, so enforce inverted
+        * semantics.
+        */
+       par->spi->mode ^= SPI_CS_HIGH;
        ret = spi_setup(par->spi);
        if (ret) {
-               dev_err(par->info->device, "Could not set SPI_CS_HIGH\n");
+               dev_err(par->info->device,
+                       "Could not set inverse CS polarity\n");
                return ret;
        }
 

diff --git a/drivers/staging/fbtft/fb_watterott.c b/drivers/staging/fbtft/fb_watterott.c
index 27cc8ea..76b25df 100644 (file)
--- a/drivers/staging/fbtft/fb_watterott.c
+++ b/drivers/staging/fbtft/fb_watterott.c
@@ -150,10 +150,17 @@ static int init_display(struct fbtft_par *par)
 
        /* enable SPI interface by having CS and MOSI low during reset */
        save_mode = par->spi->mode;
-       par->spi->mode |= SPI_CS_HIGH;
-       ret = spi_setup(par->spi); /* set CS inactive low */
+       /*
+        * Set CS active inverse polarity: just setting SPI_CS_HIGH does not
+        * work with GPIO based chip selects that are logically active high
+        * but inverted inside the GPIO library, so enforce inverted
+        * semantics.
+        */
+       par->spi->mode ^= SPI_CS_HIGH;
+       ret = spi_setup(par->spi);
        if (ret) {
-               dev_err(par->info->device, "Could not set SPI_CS_HIGH\n");
+               dev_err(par->info->device,
+                       "Could not set inverse CS polarity\n");
                return ret;
        }
        write_reg(par, 0x00); /* make sure mode is set */